1. Field of the Invention
The present invention relates to a fixing device including a separating member for separating a recording medium from a fixing rotator, an image forming device including the fixing device, and the separating member.
2. Description of the Related Art
In recent years, the market has more strongly required speed increase and energy saving for image forming devices such as printers, copiers, and facsimiles. Through an image forming process such as electronic photograph recording, electrostatic recording, and magnetic recording, an image forming device forms an unfixed toner image on a recording medium such as a recording medium sheet, a sheet of print paper, a sheet of photosensitive paper, or a sheet of electrostatic recording paper by an image transfer method or a direct method. For fixing the unfixed toner image, the fixing device often employs a contact heating method such as a thermal roller method, a film heating method, or an electromagnetic induction heating method.
As an example of such a fixing device, a belt type fixing device (see Japanese Patent Application Laid-open No. 2004-286922, for example) and a fixing device of SURF fixing (film fixing) using a ceramic heater (see Japanese Patent No. 2861280, for example) are known.
The belt type fixing device has been recently desired to have a shorter warm-up time (time required for a normal-temperature condition to reach a predetermined printable temperature (reload temperature) at a power-on time, for example) or a shorter first-printing time (time required for completing the discharge of paper after reception of a print request, preparation for printing, and printing operation) (Object 1). Along with the speed increase of the image forming device, the number of feeding sheets per unit time has increased and a larger quantity of heat has been required, resulting in a problem of shortage of the quantity of heat especially at the beginning of successive printing, which is called temperature drop (Object 2).
Meanwhile, the SURF fixing type using a ceramic heater, the heat capacity and the size thereof can be reduced as compared with the belt type fixing device; therefore, Object 1 can be solved. However, since just a nip portion of the SURF fixing type is locally heated, the other parts are not heated; the belt is the coolest at the entrance of a nip sheet or the like, which causes a problem in that fixing failure easily occurs. In particular, a high-speed machine is more likely to face a problem of higher probability of fixing failure because the belt rotation is fast and the heat release of the belt at other than the nip portion is increased (Object 3).
For solving Objects 1 to 3 as above, a fixing device has been suggested that can provide excellent fixing property even when the device is mounted on a highly productive image forming device including an endless belt (see Japanese Patent Application Laid-open No. 2007-334205).
This fixing device includes an endless belt 100, a metal thermal conductor 200 with a pipe shape installed inside the endless belt 100, a heat source 300 installed inside the metal thermal conductor 200, and a pressing roller 400 forming a nip portion N by abutting on the metal thermal conductor 200 via the endless belt 100, as depicted in FIG. 13. The rotation of the pressing roller 400 causes the endless belt 100 to rotate together, at which time the metal thermal conductor 200 guides the movement of the endless belt 100. The heat source 300 inside the metal thermal conductor 200 heats the endless belt 100 via the metal thermal conductor 200, thereby allowing the entire endless belt 100 to be heated. Thus, the first printing time from the heating stand-by time can be shortened, and the shortage of the quantity of heat at the high-speed rotation can be solved.
However, for further saving energy and improving the first printing time, the thermal efficiency needs to be improved further. In view of this, a configuration has been suggested in which the endless belt is directly (not via the metal thermal conductor) heated instead of indirectly heating the endless belt via the metal thermal conductor (see Japanese Patent Application Laid-open No. 2007-233011).
In this configuration, as depicted in FIG. 14, the pipe-shaped metal thermal conductor is removed from the inside of the endless belt 100 and a plate-shaped nip forming member 500 is formed instead at a position facing the pressing roller 400. In the case of this configuration, the endless belt 100 can be directly heated by the heat source 300 at a place other than the part provided with the nip forming member 500; thus, the heat conduction efficiency is drastically improved and the power consumption is reduced. As a result, the first printing time from the heating stand-by time can be further shortened. Moreover, the cost reduction can be achieved by omission of the metal thermal conductor.
As illustrated in FIG. 14, in the case of employing the configuration in which the endless belt is directly heated, no members exist that hold the fixing belt along the entire length (for example, the metal thermal conductor 200 depicted in FIG. 13); therefore, a problem is caused in that the radial cross-sectional shape of the fixing belt gets instable and varies in an axial direction. In this case, the gap between the fixing belt and a separating member for separating the sheet of paper having passed through the fixing nip from the fixing belt becomes inhomogeneous in the axial direction. This might lead to clogging of paper (jam) due to separation failure or a damage of the fixing belt due to the contact with the separating member, for example.
In view of this, there is a need to provide a fixing device in which the separation gap between the separating member and the fixing belt are substantially homogenized.